1. Field of the Invention
The present invention relates to apparatus for preventing electrical fires and explosions caused by the degeneration of surge protectors. More particularly, this invention pertains to a breaker circuit for a surge arrestor that is operable over a wide range of (a.c. or d.c.) power levels.
2. Description of the Prior Art
Surge arrestors are commonly employed to protect domestic electric devices, particularly on overhead supply lines leading to individual buildings, from transient or persistent overvoltages. Arrestor devices are also employed in houses supplied by underground cable as voltage surges can also occur in supply systems limited to underground cables as in those using a combination of underground cables and overhead lines. Such devices are additionally employed to protect individual electrical devices that are particularly sensitive to overvoltages.
Surge arrestors act to limit the intensity of transient or persistent overvoltages. Low voltage arrestors employed in office buildings that have electrical installations, hospitals, lighting towers and installations protected by circuit breakers prevent the undesired switching of such breakers which could otherwise be triggered by substantial overvoltages.
Surge or lightning arrestors may employ a number of devices including voltage-dependent resistors such as silicon carbide or metal oxide (MOV) varistors, silicon avalanche diodes (SAD's) and gas tubes comprising mixtures of argon and helium or other inert gas within a sealed ceramic insulating tube. Such devices act somewhat in the manner of a Zener diode, with resistance to current flow decreasing as voltage increases to limit the maximum potential difference. At very high overvoltages, large current flow through the device bleeds the voltage peak levels on the line conductor to protect the connected electrical devices, with the surge arrestor device (either alone or a parallel combination thereof) thereby acting as a voltage clamp.
Over time, such large flows of current will physically degrade the arrestor. Each time a surge occurs, some damage will take place, increasing the leakage current of the device relative to a given voltage change. After an accumulation of damage over time, excessive device leakage currents will generate enough I.sup.2 R heating to burn a hole in the device. The "end of life" of an arrestor device means it will short circuit and draw excessive current until an interruption takes place or the device explodes, burns wires and causes fires.
Catastrophic failure of a surge arrestor can be quite dangerous, producing both fires and explosions. Accordingly, attempts have been made to remove or disconnect the surge arrestor from the line conductor before the end of its useful life. Generally, such efforts have relied upon the insertion of a fuse in series with the arrestor. Such a fuse will "blow" to disconnect the device from the a.c. or d.c. circuit by sensing the amount of current being drawn.
A significant drawback of fused arrangements lies in coordination of fuse action and response with failure of the surge arrestor device. During certain events, such as thunderstorms, very large voltage transients may appear upon the line conductor. Such transients can induce large current flows that are of sufficient strength to cause the fuse to blow. However, the surge capacity of a fuse is not a direct measure of the integrity of the physical status of the surge arrestor which, as mentioned, is a function of time in the sense that damage to its current-handling capacity is cumulative. Thus, a fuse might blow despite the arrestor's ability to handle many more episodes of overvoltage. In the case of a thunderstorm, which may last many hours, electrical devices can be left unprotected against overvoltage damage for hours by a fuse's blowing as a result of a current surge occurring relatively early. Thus fuse arrangements face the inherent design contradiction posed by the need for assured activation by a low resistance short while attaining sufficient current handling capacity to prevent premature surge arrestor disconnection.
U.S. Pat. No. 5,227,944 of Andreas J. Eggendorfer for "Breaker For Surge Protector", property of the assignee herein, teaches a device that is not subject to the shortcomings of fused arrangements. Rather that relying upon the current carrying capacity of a fuse, the disclosed device operates directly in response to the physical condition of a voltage dependent resistor surge arrestor. This is accomplished by housing the varistor within an airtight housing that includes a hinged panel. An arm fixed to the hinged panel engages a mechanism which, when activated by the escape of gases from the rapidly vaporizing varistor (a varistor takes less than one second to explode in such a situation), quickly opens the circuit that connects the line conductor to ground to thereby avoid fire or explosion.
While the above-described breaker device provides a device that overcomes the shortcomings of fused arrangements, recently-issued specifications of Underwriters' Laboratories (UL 1449--"Transient Voltage Surge Suppressors, Second Edition" (1998)) have added performance requirements that cannot be addressed by the above-described device. The new UL specification requires that a breaker be capable of disconnecting a surge arrestor over a very wide range of a.c. power line currents (250 milliamps to 10,000 amperes). Currents at the lower end of the mandated spectrum will only cause a varistor (or other common surge arrestor device such as a gas tube or SAI) to overheat to a sufficient extent without activating the breaker mechanism while those at the upper end of the range can cause vaporization to take place within a fraction of a second. While the device is perfectly adequate to actuate and break the circuit prior to a fire or explosion at the upper end of the range the device will not trigger at the lower end as the I.sup.2 R heating internal to the damaged varistor will not produce the explosive disintegration of the device required to generate sufficient pressure within the airtight housing to move the panel about the hinge to actuate the breaker mechanism. Rather continuing I.sup.2 R heating may lead to significant heating of the housing that can produce fire in adjacent structures.